1. Field of the Invention
The invention comes within the field of robotics, and more precisely the field of methods of controlling a robot possessing a body supported by at least two legs on which it stands and advances.
2. Description of Related Art
Various mathematical methods for controlling the walking of such robots have been described, but they remain imperfect in many respects, given the numerous difficulties that a two-legged robot needs to overcome in order to move effectively even in an environment that is simple (e.g. flat ground without obstacles), and a fortiori in an environment that is complex, e.g. including staircases.
Certain control approaches determine a step period in one way or another. This applies for example to the disclosure of the document “Online walking motion generation with automatic foot step placement”, by Herdt et al., Advanced Robotics, 24, 719, 2010, which describes the use of predictive control to generate walking motion for a two-legged robot on the basis of a predefined reference velocity, of freely adapting the placing of the foot, and of constraints relating to the center of pressure of the robot's foot on the ground.
In that document, the robot model used is simple, but it is always for controlling the robot on the basis of a constant step period, and certain authors have developed models that are more complicated, e.g. including hip movements in order to take complex dynamics into account approximating those of a human being.
In particular, Document US 2008/0133053 addresses the stability of a robot that is subjected to a thrust. The robot is modeled by an inverted pendulum including a flywheel for reproducing the effects of angular moment modifications of the body. The authors disclose a control method operating by instantaneous determination of a position on the ground for contact of a leg with the ground, and of an angular moment of the flywheel.
In other approaches, such as for example in the article by Hyon and Fujimoto, IFAC Symposium on robot control 2009, and by Van Oort and Stramigioli, IEEE International Conference on Robotics and Automotion 2007, pp. 4653-4660, it is step length that is determined, while step duration is variable.
Proposals have also been made to use both step duration and step length as control variables. Thus, the document “Robust physics-based locomotion using low-dimensional planning” by Mordatch et al., ACM Transactions on Graphics 2010, 29 (3) discloses a controller defined on the basis of an inverted pendulum model, in which the control parameters include step duration and step location. The equations of motion are applied sequentially for at least two steps and they are polynomial equations. Control parameter continuity is imposed between two consecutive steps. Optimization on a set of at least two steps of the robot is then performed using a statistical method. In a situation without any stage in flight, control is calculated on the basis of two steps, i.e. a sequence made up of a first change of foot, a movement, a second change of foot, a new movement, and finally a third change of step.
Because such a control method applies to a set of at least two steps, it presents the drawback that calculation can require a particularly long length of time, which requires greater computer power and also reduces the reactivity of the system.
The invention seeks to solve those problems in particular, while also reducing the quantity of calculation required.